2. Process

2. Process
	Chapter 1. Introduction

Process
Main concept in a operating system: is a running program abstraction.
Figure 1.2. Processes
Pseudo parallelism: The CPU is switched between processes, given the illusion of parallelism.
Process Model
- All programs, including the operating system, are organized in a sequential process set.
- In a concept model, each process has its own CPU.
- Multiprogramming is used to switch the CPU between processes.
  Figure 1.3. Process Model
Attention:
- Depending on how the CPU is switched, process execution time vary.
- Execution at different moments may results in different execution time.
- In UNIX, time considerations for use in real-time applications are not reliable.
Analogy
- Computer scientist making a cake.
  - RECIPE: program
  - COMPUTER SCIENTIST: CPU
  - INGREDIENTS: input data
  - PROCESS: activities:
    read the recipe
    find the ingredients
    bake the cake.
- Interruptions are possible: son is attacked by bee and run into kitchen.
  - Computer scientist records his stage in the recipe.
  - Take a first emergency book (another program)
  - Follow the instructions (another program)
Process States
Figure 1.4. Process States
- 1. Process block for input
- 2. Scheduler searches for another process
- 3. Schedule picks a process
- 4. Input becomes available
Implementation
Figure 1.5. Process Implementation
1. Device (disc) starts an interruption
2. Hardware puts on stack:
  1. Program Counter
  2. Program status word
  3. registers
3. CPU jumps to the specified address to serve the interruption
4. Attendant service save all register (assembly)
5. Attendant service runs the service
6. Attendant service finds which process requested the service
7. The process state is changed from blocked to ready
8. The scheduler is called to select next process
9. The control returns to the attendant service
10. The registers are reload
11. The process restarts
A Process Consists of:
- An executable program
- Data and program stacks
- A program counter
- A stack pointer
- Registers
Preemptive Multitasking: the OS periodically decides to stop the execution of a process and starts the execution of the another process. Eventually, the stopped process must come back to the execution point where it was when it was interrupted.
All process information must be explicitly stored. Process tables implement this.
Process tables implement this.
A Suspend Process Consists of:
- Its address space (core image)
- Its process table (registers)
Example:
- a shell process reads commands (from terminal)
- a user inputs a command (Ex: program compilation)
- the shell starts a process (child) to run the compiler
- this process runs the compilation and activates a System Call to kill itself
- control goes back to the shell process.
```
while (TRUE) { 

    read_command (command, parameters); 

    if (fork() != 0){ 

      wait (&status); 

    } else 

   execve (command, parameters, 0); 

  } 
```
Process can create processes
Figure 1.6. Process Tree
Other process System Calls:
- Memory request
- Wait a child process termination
- Memory reallocation
Signals
Signals are OS mechanisms to notify events to processes. A signal can suspend, in a temporary way, process execution, save his registers on stacks and start a special program to treat it.
Ex: time out, memory violation, etc.
UID
- Each authorized person to use the system has a UID (user identification).
- Each started process has the UID from the user that started it.
- The super-user has a special UID. This UID can violate the protection rules.
Files
The second bigger set of System Calls works with the FILE SYSTEM.
Main function of the OS: hide particularities from discs and other I/O devices. The programmer must have an abstract model of the file system. This model must be independent of file system devices implementation.
Directory tree is a UNIX key concept, used to group files:
Figure 1.7. File System Tree
The processes and files hierarchy are both organized in trees. However, they have many differences:
- Process hierarchies are not very depth, but file hierarchies are.
- Process hierarchies have a short life. File hierarchies can last for years.
- Normally, child processes can be accessed or modified only by their parent process, while files can be accessed by anyone.
- Each file, in a directory hierarchy, can be specified using its pathname.
- A "/" on the beginning of a file pathname indicates that the path is absolute.
- On each moment, each process has a working directory, used to find processes' files without absolute pathnames.


1. Operating Systems		3. Memory Management